Nice bike. I have an older Mirage (with steel rims) that is an enjoyable ride despite the wheels. I am sure the aluminum rims on your bike will improve things even more.

With respect to your seat post and stem, I suggest that you get a suitable seat post first, and set the seat at the right height and fore-aft placement. Then calculate what length of stem you will need for the bike to fit your torso. As for the seat post, make absolutely sure that the diameter is in fact 26 mm and hunt around on eBay for a nice used one, or on Amazon for a new one. I have used Kalloy posts on my French bikes, and they look a bit clunky but work well and can be found in unusual diameters. But they are not the only choice!

For the stem, measure that as well. The old French standard was 22 mm diameter, but the more modern standard is 22.2 mm. Motobécane started using the modern standards sooner than the other French manufacturers, so your existing post might be 22.2, but in any case you need to check. It is pretty easy to sand down a modern post to fit (I have done it a few times) but it is possible to find a French post on eBay as well. Oh, and measure the bars as well where they fit inside the stem. French bars typically were 25 mm in diameter at the clamp, which is smaller than modern bars.

If you choose to remove the suicide levers, you can easily just hacksaw off the extension on the barrel. I have done that for several old Dia-Compe and Weinmann levers.

Finally, consider getting a set of Suntour Handlebar end shifters (aka Barcons). They are much nicer than stem shifters.